const int trigPin = 9; 
const int echoPin = 10;
const int speakerPin = 5;

float duration, distance;

void setup() {
  pinMode(trigPin, OUTPUT); 
  pinMode(echoPin, INPUT);
  pinMode(speakerPin, OUTPUT);  
  Serial.begin(9600); 
}

void beep (unsigned char speakerPin, int frequencyInHertz, long timeInMilliseconds)
{
    int x;
    long delayAmount = (long)(1000000/frequencyInHertz);
    long loopTime = (long)((timeInMilliseconds*1000)/(delayAmount*2));
    for (x=0;x<loopTime;x++)
    {    
        digitalWrite(speakerPin,HIGH);
        delayMicroseconds(delayAmount);
        digitalWrite(speakerPin,LOW);
        delayMicroseconds(delayAmount);
    }
}        

void loop() {
 digitalWrite(trigPin, LOW); 
 delayMicroseconds(2); 
 digitalWrite(trigPin, HIGH); 
 delayMicroseconds(10); 
 digitalWrite(trigPin, LOW);

 duration = pulseIn(echoPin, HIGH); 
 distance = (duration*.0343)/2;

 int tonhoehe = 1600-(distance*10);

 if (distance < 150) {
  beep(speakerPin, tonhoehe, 100);
 }

 if (distance >= 150) digitalWrite(speakerPin,LOW);

 if (distance < 550) {
  Serial.print("Entf [cm]: "); 
  Serial.println(distance); 
 }

 delay(10);
}